Convergence, political and evolutionary

Among the papers in the archives of Harry Holland which I have been looking at over the past week in Canberra, I found a talk he gave in the early 1920s to an audience of workers entitled “Darwin and Marx: the world’s greatest revolutionaries.”  My admiration grew for this self-educated fighter, whose formal education began and ended in a tin shed primary school in the Australian backblocks, yet who had tackled both Origin of Species and Capital.

Australia is a great place to think about Darwinian evolution. Its marsupial fauna, which expanded after the extinction of the dinosaurs to fill every niche occupied by placental mammals in Eurasia-North America, provides many examples of parallel and convergent evolution. The slow-moving, long-sleeping,  leaf-eating sloth of America, a placental mammal, has its counterpart in the slow-moving, long-sleeping, eucalyptus-leaf-eating marsupial koala in Australia.

sloth

Panamanian sloth

Koala

koala – marsupial equivalent of sloth

At times this convergence confers a striking physical similarity. The now-extinct marsupial Tasmanian tiger had an extraordinarily dog-like appearance, and filled a similar ecological niche to the Eurasian wolf.

tasmanian_tiger

the last known tasmanian tiger, 1936

I find it interesting how horse-like is the head shape of the big red kangaroo, despite the obvious dissimilarity in the rest of its body. Both horse and kangaroo have large, upright, freely-pivoting ears, eyes set well out on the side of the head, and prominent upper incisors.

horse grazing  red-kangaroo kangaroo grazing

What is the reason for this similarity? Not closeness of ancestry: the kangaroo is more closely related to the koala, with its very different head shape.  The physical similarity comes from their evolutionary adaptation to a similar way of life: both horse and kangaroo are grazers. They spend much of the day with their heads down, eating grass (hence the need for large upper incisors for tearing the tough grass). In this position, they are vulnerable to predators, hence the need for large pivoting ears, and eyes protruding sufficiently from the side of the head that they can see movement in a wide arc of view.

I find this evolutionary convergence one of the most satisfying kinds of evidence for Darwinian evolution.

Harry Holland was not mistaken. Darwin was a revolutionary thinker, and a hundred years after Holland gave his talk, defence of Darwinian evolution remains a touchstone  of scientific and materialist thinking.

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